(1) Field of the Invention
This invention relates to a porous regenerated cellulose hollow fiber having a novel structure including pores, in which the average pore diameter on the inner and outer wall surfaces is 0.02 to 10 .mu.m, and also to be process for the preparation thereof. More particularly, the present invention relates to a porous regenerated cellulose hollow fiber, which is characterized in that the viscosity average molecular weight of cellulose molecules forming a wall thickness portion is at least 5.times.10.sup.4, each of the average pore diameters of the outer wall surface and inner wall surface of the hollow fiber is 0.02 to 10 .mu.m, the in-plane porosity Pr of the outer wall surface is at least 10%, and the hollow fiber comprises a wall thickness portion having pores extending therethrough between the inner wall surface and the outer wall surface and a hollow portion extending continuously over the entire fiber length. The present invention also relates to a process for the preparation of a porous regenerated cellulose fiber having a screen filter structure and a hollow portion extending continuously through the entire fiber length, characterized in that at the steps of extruding a cuprammonium solution of a cellulose having an average viscosity of at least 5.times.10.sup.4 from an annular spinning orifice and coagulating, regenerating and water-washing the extrudate, the spinning solution is extruded from an outer annular spinning orifice and a hollowing agent is extruded from a central spinning orifice, a liquid having a coagulating action to the spinning solution is used as the hollowing agent, and micro-phase-separation is caused to occur in the fibrous extrudate before coagulation. By the term "screen filter structure" used in the instant specification is meant a structure in which pores having a size of at least 0.02 .mu.m are found on the entire surface of the wall thickness portion when the wall thickness portion is observed by an electron microscope. In case of this screen filter structure, it is confirmed by the electron microscope observation that pores are present in both the inner and outer surfaces of the wall thickness portion. A hollow fiber having such pores that can be observed by an electron microscope is called "a porous hollow fiber", and a hollow fiber in which the presence of such pores cannot be confirmed by an electron microscope is called "a non-porous hollow fiber".
(2) Description of the Prior Art
Of techniques of separating and purifying substances, the membrane separation technique has intensively been studied as means for separating ions, low-molecular-weight substances and substances having a size of the micron order such as suspended substances and fine particles in a liquid phase. One most difficult problem inhibiting practical application of this technique on a commercial scale is a low speed of separating substances. The substance-separating speed depends on the area of a membrane used. Accordingly, the membrane area should be increased with an increase of the amount of the substance to be treated, and in case of a plane membrane, the size of the apparatus is inevitably increased. This problem is solved by increasing the effective area of the separating membrane per unit volume and reducing the size of the apparatus by forming a plane membrane by very fine hollow fibers, performing separation of substances through fiber walls surrounding hollow portions as the separating membrane and constructing a substance-separating zone by bundling a plurality of hollow fibers. As the field where it is expected that the membrane separation system will be a main system in the future, there can be considered (1) the field where concentration, purification and recovery at low temperatures are necessary (such as fields of foodstuffs and biochemical industries), (2) the field where sterile and dust-free conditions are necessary (such as pharmaceutical and medical industries and electronics industries), (3) the field where concentration and recovery of minute amounts of expensive substances should be performed (such as atomic and heavy metal industries), (4) the field where minute amounts of special substances are separated (such as pharmaceutical and medical industries, and (5) the field where a large quantity of energy is consumed (such as substitute means for distillation). Hydrophilic membranes having a large pore size, which are capable of being handled very easily, are desired as membranes to be used in these fields.
A regenerated cellulose can be mentioned as a material having a high hydrophilic property. The solubility parameter .delta..sub.h depending on the hydrogen bond can be adopted as a value indicating the hydrophilic characteristic. The solubility parameters .delta..sub.h of a regenerated cellulose, a PVA copolymer, cellulose acetate, polymethyl methacrylate, polyacrylonitrile, polyethylene, polypropylene and Teflon are 11.9, 4.1 to 11.7, 6.6, 4.4, 3.7, 0.0, 0.0, and 0.0 (Cal/ml).sup.1/2, respectively. Among these polymers, the regenerated cellulose has the highest solubility parameter. The regenerated cellulose is insoluble in most of organic solvents and is excellent in the organic solvent resistance. Because of this high organic solvent resistance, a hollow fiber having a large average pore size has not been prepared from the regenerated cellulose. As another excellent property of the regenerated cellulose, there can be mentioned a high glass transition temperature (at least 150.degree. C.) in the dry state or in an organic solvent. Furthermore, the regenerated cellulose is different from other synthetic polymers in the point where it has no toxicity to living bodies. Accordingly, development of a porous membrane of a regenerated cellulose having a large average pore size has been desired in the field of ultrafiltration using membranes.
As the hollow fiber composed of a cellulose as a typical instance of a hydrophilic polymer, there is known a hollow fiber for an artificial kidney (so-called non-porous hollow fiber in which the cross-sectional and longitudinal-sectional surfaces have fine pores having a size of up to 200 .ANG. (0.02 .mu.m) (see Japanese Unexamined Patent Publication No.49-134,920). Since this hollow fiber has a small pore size and a small average porosity Pr.rho. (8%), it can hardly be used for ultrafiltration or microfiltration.
Furthermore, a process for preparing a regenerated cellulose hollow fiber by saponifying a cellulose derivative such as cellulose acetate or cellulose nitrate with an aqueous solution of an alkali is known (see U.S. Pat. No. 4,219,517). In the hollow fiber obtained according to this process, the average pore diameter can be adjusted to 0.01 to 2 .mu.m. However, since a cellulose derivative is used as the starting material, the average molecular weight of the cellulose molecules after regeneration is lower than 3.5.times.10.sup.4 and the hydrophilic property is lower than that of a regenerated cellulose obtained according to the cuprammonium process. Accordingly, the mechanical properties (especially the strength) of this hollow fiber in the dry state are much lower than those of the conventional regenerated cellulose hollow fiber, and this hollow fiber is brittle. For example, the tensile elastic modulus of this hollow fiber is about 10.sup.2 (100-Pr.rho.).sup.3 dyn/cm.sup.2. The tensile strength at break is substantially in proportion to the elastic modulus and is about 1/10 of the elastic modulus. The strength in the state wetted with water is much lower than the strength in the dry state. Accordingly, the conventional regenerated cellulose hollow fiber obtained from a cellulose derivative is often broken during handling. Furthermore, in the above-mentioned process for the preparation of a regenerated cellulose hollow fiber which comprises regenerating a cellulose derivative, the preparation steps become complicated and the manufacturing cost is increased.
As typical instances of the non-porous regenerated cellulose hollow fiber prepared from a cuprammonium solution of a cellulose, there can be mentioned (1) a cuprammonium cellulose hollow fiber, which has a uniform wall thickness of several to 60 .mu.m along the entire fiber length and entire circumference and a uniform cross-section of a true circle having an outer diameter of 10 to several hundred .mu.m and also having as hollow portion which extends continuously over the oriented entire fiber length (see Japanese Examined Patent Publication No. 50-40,168), (2) a hollow artificial fiber composed of a cuprammonium regenerated cellulose which has a cross-sectional structure in which the portion close to the outer surface has a denser porous structure than the portion close to the inner surface and the intermediate portion (see Japanese Examined Patent Publication No. 55-1,363), and (3) a hollow fiber for the dialysis, which is composed of a cuprammonium regenerated cellulose and has skinless smooth inner and outer surfaces, said fiber having a substantially uniform, dense and porous structure in which by electron microscope observation of a cuprammonium regenerated cellulose tube having a hollow core in the wet state, it is confirmed that fine pores having a size of up to 200 .ANG. are present entirely on the cross-sectional surface and longitudinal-sectional surface (see Japanese Unexamined Patent Publication No. 49-134,920). Each of these known hollow fibers is prepared by directly extruding a cuprammonium cellulose spinning solution into air or a non-coagulating fluid through an annular orifice of a spinneret, causing the extrudate to fall down by the action of gravity or to rise by utilizing buoyant force, guiding a liquid having no coagulating action to the spinning solution to the inner central portion of the extruded spinning solution and extruding the liquid into the spinning solution, sufficiently drawing the extrudate by falling owing to the action of gravity or rising owing to buoyant force, and dipping the extrudate into an aqueous dilute solution of sulfuric acid to effect coagulation and regeneration. All of the hollow fibers prepared according to this process have an average pore diameter smaller than 0.02 .mu.m. Accordingly, these hollow fibers cannot be applied to production of pure water, concentration and purification of foods, purification of medicines, sterilization or removal of fine particles. Therefore. development of a hollow fiber having a large pore diameter has been eagerly desired.